The present invention relates to a solid oxide fuel cell stack having thermally responsive stack compression elements.
In a solid oxide fuel cell, oxidant and fuel are electrochemically reacted to produce electricity. The reactants are supplied to the cell through manifolds and channels that direct the reactants to the appropriate sides of a solid ceramic membrane that acts as an electrolyte. The membrane is coated with electrodes on both sides, and is impervious to the transfer of electrons, but not ions of the reactants. Thus the streams of reactants are kept separate, but the electrons and ions from the reactants are allowed contact to effect the reaction. During operation electrons are emitted at the fuel side electrode (anode) of the solid electrolyte membrane whereas electrons are absorbed at the oxygen side electrode (cathode), generating a potential difference between the two electrodes. The solid electrolyte membrane separates the reactants and transfers the charge in the form of ions. At the same time, the electrolyte prevents an electrical short circuit between the two electrodes of the solid electrolyte. For this purpose, the solid electrolyte membrane needs to have a low conductivity for electrons but at the same time, a high conductivity for ions through the vertical cross section of the membrane.
A fuel cell stack is made from a plurality of interleaved fuel cells and interconnect plates which act as barriers between the anode of one cell and the cathode of the adjacent cell. Each individual interconnect plate is sealed to the adjacent interconnect plates, and in addition each fuel and oxidant manifold within the interconnect plate is individually sealed. The seals are necessary to prevent mixing of fuel and oxidant gases. In order to enhance the sealing efficiency of the seals, it is desirable to compress the entire stack. If stack compression is not maintained, the seals may leak and allow fuel and oxidant to mix. Because the fuel cell typically operates above the autoignition temperatures of the fuel gases, a fuel leak may be disastrous.
Solid oxide fuel cells typically operate at high temperature, often in excess of 600xc2x0 C., which limits the selection of materials available for use as stack components, such as interconnect plates and stack compression devices. The operating conditions inside a fuel cell environment are harsh and require materials that have high heat creep resistance such that the stack compression pressure can be maintained.
Prior art compression devices have either been external devices that operated outside the high heat zone or have used expensive exotic materials. The external compression devices are bulky, and are unsuitable for space restricted applications such as auxiliary power generation in automobiles.
Accordingly, there is a need in the art for a device that can provide the required level of stack compression, yet be compact and made from more inexpensive materials than the prior art.
The present invention is directed at a fuel cell stack having means for applying a compressive force to the stack, wherein the compressive force remains substantially the same through the operating temperature range of the fuel cell stack, such that seal integrity is maintained through thermal cycling that a fuel cell stack experiences in operation. When a fuel cell stack is assembled at ambient temperatures, a pre-load compressive force may be applied by tie rods which extend through the stack and which are secured to the top and bottom plates of the stack. As the fuel cell stack increases in temperature, the stack expands in accordance with its coefficient of thermal expansion (xe2x80x9cCTExe2x80x9d) which will be a composite CTE of the layered components of the stack. If the tie rods have a greater CTE than the composite CTE of the stack, the compressive force will decrease as the stack temperature increases. If the tie rods have a CTE substantially equal to the composite CTE of the stack, the compressive force will still decrease because of high temperature creep of the tie rods. The higher the creep resistance of the tie rods, the more constant the compressive force will be. If the CTE of the tie rods is less than the composite CTE of the stack, and the tie rods have significant creep resistance, the compressive force on the stack may actually increase as the stack temperature increases.
It is desirable that the stack compressive force remain relatively constant through its thermal cycles. If the force significantly decreases, the seals within the stack may develop leaks. If the force significantly increases, components within the stack may crack or develop other structural problems.
In one aspect of the invention, in a solid oxide fuel cell stack including a bottom plate and top plate, the tie rod may actually be an elongate compression device instead of a unitary tie rod. The compression device may comprise:
(a)a first compression member attached to the top plate and having a lower upward facing shoulder;
(b)a second compression member attached to the bottom plate and having an upper downward facing shoulder;
(c)an expansion member disposed between the first compression member shoulder and the second compression member shoulder;
(d)wherein said expansion member has a coefficient of thermal expansion greater than or substantially equal to one or both of the first and second compression members.
In another aspect, the invention may comprise a solid oxide fuel cell stack including a bottom plate and top plate and a thermally acting compression device comprising:
(a)an elongate outer sleeve attached to the top plate and having an inwardly protruding shoulder formed at a lower end;
(b)an elongate inner sleeve contained within the outer sleeve;
(c)a tie rod attached to the bottom plate and extending through the inner sleeve, said tie rod having a retaining ring affixed thereto at an upper end of the tie rod;
wherein the length of the inner sleeve is substantially the same as the distance between the lower shoulder of the outer sleeve and the retaining ring; and wherein the inner sleeve comprises a material having a first coefficient of thermal expansion and one or both of the outer sleeve and tie rod comprise a material having a second coefficient of thermal expansion, which may the same or different from the first CTE, and the first and second CTE""s are chosen such that the compressive force applied by the compression device remains substantially the same through the fuel cell stack thermal cycle.
Preferably, the first collar is mounted flush with the top plate and the second collar is mounted flush with the bottom plate.
In another aspect, the invention may comprise a solid oxide fuel cell stack comprising a plurality of fuel cells interleaved with interconnects, a bottom plate and a top plate and stack compression tie rods, wherein the coefficient of thermal expansion of the tie rods and remainder of the compression apparatus is substantially the same or less than the composite extending between the top plate and bottom plate coefficient of thermal expansion of the stack. In one embodiment, the stack compression tie rods are each comprised of a lower rod which may be slidingly displaced within an upper sleeve, and an expansion member fitted between shoulders formed on each of the lower rod and upper sleeve. Preferably the expansion member has a higher CTE than the lower rod and upper sleeve.